There are 2 main principles used in the design of steel members and connections.
Allowable Strength Design - AISC 360-16 ASD
Load Resistance Factor Design - AISC 360-16 LRFD
Structural Safety
The increment of loads that act into the structure with certain coefficients and decreasing the capacity of the structural members with certain coefficients are specified by the regulations. Different loads (uniform, concentrated, live, snow, wind, earthquake, etc.) are multiplied by the coefficients for the cases when the load computation is done incompletely. The capacity of the member is determined according to the cross-section and material properties, which is reduced by certain coefficients in order to take into account the deviations in material properties, manufacturing defects, etc.